So, from a pure technical point of view, is it possible to change
values of e(b)?

I believe not. You can read from it but not
write to it directly.

That is certainly untrue; estimates post does that for a living. That is,
write a program that is a wrapper for your estimation command, grab the
e(b) and e(V) into matrices within the routine, and use estimates post to
make them the active e(b) and e(V).

This program will change the specified element of the coefficient vector to
the specified value. After it is run, e(b) will contain the altered
coefficient vector. A well-written variant would check the appropriateness
of coef before attempting to modify the b matrix.